1. Field of the Invention
The present invention relates to an improved process for preparing N-acylamino carboxylic acids and N-acylamino sulfonic acids and their alkali metal salts from the technical alkali metal salts of amino carboxylic acids and amino sulfonic acids, respectively, and from alkyl carboxylates.
2. Discussion of the Background
DE-A 20 04 099 discloses that salts of N-acylamino carboxylic acids can be prepared by acylation of the corresponding amino carboxylic acids or their salts with a carboxylic acid, a carboxylic ester or a carboxamide in the presence of an equivalent amount of a basic compound which forms salts with carboxylic acids, such as an alkali metal or alkaline earth metal hydroxide, at from 100.degree. to 250.degree. C., preferably from 160.degree. to 200.degree. C. In this case the basic compound merely serves to neutralize the amino carboxylic acids or carboxylic acids used. The reaction is carried out in the melt of amino carboxylic acid and acylating agent or in a suspension of the amino carboxylic acid in a solution of an acylating agent and of a basic organic nitrogen compound, eg. an amine, in a high-boiling organic solvent.
However, the disadvantages of the described reaction procedure are the long reaction times and the low yields, which are attributable essentially to the partial decomposition of the starting compounds and of the products because of the high reaction temperatures. This is because the reaction mixture becomes slightly dark in color during this and a slight evolution of carbon dioxide which is almost always observed can likewise be explained only by decomposition reactions.
German Patent Application P 44 08 957.0 describes a process for preparing N-acylamino carboxylic and sulfonic acids from the alkali metal salts of the underlying amino carboxylic and sulfonic acids, respectively, and alkyl carboxylates, in which 0.5 to 30 mol % of strong bases are added as catalyst to a suspension of the amino carboxylic and sulfonic acid salts, respectively, in the carboxylates. However, this process gives satisfactory results only on use of pure alkali metal amino carboxylates or sulfonates with an active content of more than 95% by weight, based on the solids content of the alkali metal salts. Use of corresponding salts of technical quality results in drastic losses of selectivity of up to 15%.